Devices for electrically stimulating tissue, including cochlear implants, pacemakers, muscle and spinal cord stimulators, have been in use for decades. Retinal prostheses that might assist some of the estimated 10 million people worldwide who are blind as a result of degenerative retinal diseases, such as age-related macular degeneration and retinitis pigmentosa, are under development based on the concept of replacing photoreceptor function with an electronic nerve-stimulating device.
Many tissue-stimulating prostheses provide electrical signals to an implanted section, which then generates excitation signals to excite the tissue of a patient by means of appropriately positioned stimulation electrodes or arrays of electrodes. Common in some tissue stimulators is a two-part design, wherein an external section transmits RF energy that is inductively coupled by a transcutaneous RF link to the implanted section. The energy of the coupled RF electrical signals is rectified and stored by a power supply located in the internal section. It is that power supply that provides the energy required to power the internal section and to generate the stimulus signals.
To increase patient safety, to minimize the power requirements of a tissue stimulator, and because power dissipation losses are in proportion to the square of the voltage, it is desirable to operate the tissue stimulator at low voltages that are no greater than required. This is especially true in implantable stimulator electronics. During stimulation, the electrodes of tissue stimulating devices are typically driven by a constant current source to a prescribed level of charge, resulting in storage of that charge, and therefore storage of energy, within the electrodes. To ensure charge balancing to avoid electroplating the electrodes, the same amount of charge is then driven in the opposite direction (changing polarity) until the electrodes are left uncharged. Typical circuit techniques are very inefficient when driving this type of electrode, often using more than twice the necessary energy during the first phase of current drive, and even more during the second phase.
U.S. Pat. No. 5,522,865 to Schulman, et al., entitled “Voltage/Current Control System for a Human Tissue Stimulator” discloses a human tissue stimulating system that comprises an audio responsive system for artificially stimulating a cochlea to improve hearing for the hearing impaired. The implanted stimulator includes a power supply that extracts raw power from a data signal, a voltage downconverter for providing a number of output voltages from the extracted raw power signal, and a storage capacitor that serves as the power source for portions of the stimulator. One of the output voltages is applied to isolated refresh voltage capacitors, where it controls a voltage controlled current source that supplies output to the electrodes through a complex switch matrix. Energy is conserved by turning off and on various subsystems within a control processor, and by optimizing power dissipation of a conventional input switching regulator by controlling the RF power transmitted from an external source to the implanted stimulator based on a telemetered voltage drop across the regulator, indicating what power is required to be transmitted for just sufficient stimulator operation.
U.S. Pat. No. 5,876,425 to Gord, et al., entitled “Power Control Loop for Implantable Tissue Stimulator” also describes a feedback power control loop utilizing back telemetry from the implantable device. The voltage level of a tank capacitor utilized as an internal rechargeable power source is transmitted to an external power supply processor for computation and delivery of an appropriate amount of power to maintain normal operation, while minimizing transmission of extra energy that might otherwise be dissipated.
U.S. Pat. No. 6,415,186 to Siu-Chor Chim, et al., entitled “Active Feed Forward Power Control Loop” discloses a feed forward power control loop for providing power to the implanted part of a tissue stimulator. Power consumption is similarly kept low by transmitting across a wireless transcutaneous transmission link only the amount of power required by the implanted device, as predicted by the power control loop processor. The reference discloses the use of intermittent telemetry and predictive modeling to determine the appropriate amount of power to transmit.
Each of the references cited above approaches power transfer optimization by using tank capacitor voltage telemetry to determine power transmission. They address power consumption efficiency of the implanted circuitry, to a greater or lesser extent, by turning on and off circuit components, and otherwise treat conventionally the transfer of energy from the power supply to the electrodes. None address recovering energy from the electrodes and other components after stimulation has occurred.
U.S. Pat. No. 6,181,969 to Gord, entitled “Programmable Current Output Stimulus Stage for Implantable Device” discloses a programmable output current source for use within an implantable tissue or nerve stimulator. Each electrode node has parallel-connected P-FET current source sets permanently connected between it and a positive voltage rail, and parallel-connected N-FET current source sets permanently connected between it and a negative voltage rail. The higher power requirement of the PFET and NFET current sources is kept to a minimum by avoiding physically or electrically “switching” the electrode nodes between one or more circuit locations, or to different sides of a current or voltage source so as to change the polarity of the current flowing through the node. Rather, the P-FET sources “source” current to the node, and the N-FET sources receive, or “sink”, current from the node. Such a non-switching approach is achieved at the cost of more circuit components.
Accordingly, there is a continuing need for greater power utilization and delivery efficiency in tissue stimulating devices. The present invention satisfies such needs.